Continuous process for producing N-butanol employing anaerobic fermentation

ABSTRACT

There is disclosed, in one aspect, a continuous process for producing n-butanol. This process comprises (a) continuously contacting at least one carbohydrate-containing substrate, such as blackstrap molasses, with an n-butanol producing culture, such as Clostridium acetobutylicum (Weizmann), in water to effect the fermentation of the substrate and form a product mixture comprising n-butanol, (b) continuously extracting the product mixture from the substrate, culture, and water by forming a solution of the product mixture with an extraction solvent, such as monofluorotrichloromethane, (c) continuously separating the extraction solvent from the product mixture by vaporizing substantially all of the solvent without substantial vaporization of the product mixture, and (d) continuously condensing the vaporized solvent for reuse as an extraction solvent in step b. In another aspect, there is disclosed an apparatus for conducting such a process.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 076,250, filed Sept. 17, 1979, now abandonedentitled "Method For Continuous Anaerobic Fermentation Of Carbohydrates,Sugars, And The Like To Produce Solvent Materials Such As N-Butanol AndApparatus For The Continuous Anaerobic Fermentation Process", thedisclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Normal (n)-butanol and acetone may be produced by anaerobic fermentationprocesses as disclosed by Weizmann. Such processes were firstcommercialized during the period 1914 to 1918. The cultures which weredeveloped and discovered by Weizmann are the anaerobic bacteriaClostridium Acetobutylicum Weizmann. Using feed stock materials such ascorn and horse chestnuts, these bacteria produce n-butanol and acetonein commercial yields. During World War I, the process was used primarilyfor obtaining acetone for the manufacture of explosives.

Over the years, improvements were made in the process. There weredeveloped different Clostridium cultures which produced both betteryields and different mixtures of solvents including ethanol andpropanol. Different feedstock materials such as corn cobs, blackstrapmolasses, beet molasses, and others were also used. Furthermore, therewere employed other additives as well as nutrients which speeded up thefermentation reaction. The process was operated on a commercial scaleuntil the middle 1950's when cheap petroleum feed stocks as well asprocesses for making butanol and the other solvents from petroleumbecame available and petroleum became the primary source for producingbutanol.

These fermentation processes have become of greater interest of late dueto the recent sharp increase and apparent long term rise in the price ofpetroleum, coupled with the stable price of farm product feeds for thefermentation. Fermentation products are of current commercial interestas a potential internal combustion fuel to replace the petroleum basedfuels. A problem exists, however, with the Weizmann fermentation processbecause the maximum concentration of butanol is of the order of 2.5%before the bacteria are inactivated. The distillation recovery methodsused to separate these solvents requires a tremendous amount of energy.

The search has continued for improved processes and apparati forproducing n-butanol on a continuous, economical basis. This inventionwas made as a result of that search.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, is is a general object of the present invention to avoid orsubstantially alleviate the above-noted problems.

A more specific object of the present invention is to provide acontinuous process for the production of butanol and other solvents bythe anaerobic fermentation of substrates such as sugar andstarch-containing agricultural products.

Another object of this invention to provide an apparatus forcontinuously passing a fermentation liquor through a stationary culturebed.

It is a further object of this invention to provide a reactor structurewhich will allow the removal of fermentation products from thefermentation liquor during continuous processing so that the culturemedium will not be inactivated.

Still another object of the invention is to provide means for extractinga desired product from a fermentation liquor during the fermentationprocess without expending substantial energy.

A further object of the invention is to provide a reactor system whichwill provide for the continuous recycle of a solvent system andfermentation liquor without requiring more than small make-up amounts ofwater and solvent.

Yet another object of the invention is to provide means for makingn-butanol at low cost for use as a fuel for internal combustion engines.

Other objects and advantages of the invention will become apparent fromthe following summary of the invention and description of its preferredembodiments.

The present invention provides, in one aspect, a continuous process forproducing n-butanol from the starting materials of an anaerobicfermentation process. This process comprises (a) continuously contactingat least one carbohydrate-containing substrate with an n-butanolproducing culture in water to effect the fermentation of the substrateand form a product mixture comprising n-butanol, (b) continuouslyextracting the product mixture from the substrate, culture, and water byforming a solution of the product mixture with an extraction solventwhile substantially avoiding the formation of a solution of the solventwith the substrate, culture, and water, (c) continuously separating theextraction solvent from the product mixture by vaporizing substantiallyall of the solvent without substantial vaporization of the productmixture, and (d) continuously condensing the vaporized solvent for reuseas an extraction solvent in step b.

In another aspect, the present invention provides an apparatus forconducting a continuous anaerobic fermentation of at least onecarbohydrate-containing substrate with a culture to form a reactionproduct mixture. The apparatus comprises (a) means for holding thesubstrate and water, (b) reactor means for holding the culture, andfacilitating contact between the substrate and culture, (c) means forfeeding the substrate from the holding means to the reactor means forreaction with the culture to produce the reaction product mixture, (d)extraction means for contacting the reaction product mixture with anextraction solvent to form a solution of the solvent and productmixture, while substantially avoiding the formation of a solution of thesolvent with the substrate, culture, and water, (e) means for vaporizingthe extraction solvent while substantially avoiding the vaporization ofthe product mixture, and (f) means for condensing the vaporizedextraction solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is described further in accordance with the devices,materials, constructions, combinations, and arrangements of parts shownby way of example and illustrated in the accompanying drawings of apreferred embodiment in which:

FIG. 1 is an elevation view, partly in section, of a reactor foranaerobic fermentation of carbohydrate and sugar materials.

FIG. 2 is a section through the view of FIG. 1 which shows the generalarrangement of the flow structure in the main body of the fermentationreactor.

FIG. 3 is an enlargement in section of the central pipe in the main bodyof the fermentation reactor.

FIG. 4 is a sectional view of the membrane wall of the main fermentationreactor body.

FIG. 5 is a view into the section of FIG. 4 showing the construction ofa membrane support.

FIG. 6 is an alternative construction of the membrane wall of thefermentation reactor which is designed to reduce the resistance to flowof the fermentation liquor.

FIG. 7 is a plan view of FIG. 6.

FIG. 8 is an alternative arrangement of the extractor unit using lowdensity extraction solvents.

FIG. 9 is a partial section of the main reactor body with additionalmembranes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, like parts are designated by like reference numeralsthroughout.

Referring first to FIG. 1, the reference numeral 10 refers to afermentation system made in accordance with the present invention. Thefermentation system comprises four main units: fermentation reactor 11,extraction unit 12, feed tank 13, and extraction separation unit 14.Reactor 11 is connected to feed tank 13 by means of pipelines 15 and 16which are connected by pump 17. Pipe line 16 is connected into centraldistribution pipe 18 of reactor 11. Central distribution pipe 18 has aporous or perforated wall 19 shown in FIG. 3. This perforated or porouswall extends from liquid level 20 (FIG. 1) to reactor bottom 27 at thepoint designated as 21.

FIG. 2 illustrates that reactor 11 comprises several concentric walls.Central distribution pipe 18 is sealed to reactor bottom 27 at the pointdesignated as 21. Membrane wall 22 comprises filter medium 23 as shownin detail in FIG. 4. Filter medium 23 is supported by structural mesh 24as shown in detail in FIG. 5. Outer wall 26 of reactor 11 is asolid-liquid retaining wall as is reactor bottom 27.

Reactor 11 is connected by means of pipe line 28 to extractor unit 12.The point of entry of pipe line 28 into extractor unit 12 is on thelower portion of extractor body 30, designated as 29. Extraction unit 12is connected by means of pipe line 31 to the extraction separation unit14 which has a set of injector units 32 attached to standing pipe 33which runs down unit 12. Standing pipe 33 is connected by pipe line 34to compressor 35. Compressor 35 is connected by pipe line 36 toextraction separation unit 14. Pipe line 34 has a valved make-up port 37for introducing additional extraction solvent.

Extraction unit 12 is connected to feed tank 13 by means of pipe line38. Feed tank 13 is equipped with a valved entry 39 for new fermentationmaterials. Pipe line 31 which connects extraction unit 12 to extractionseparation unit 14 is terminated inside separation unit 14 via nozzlesection 40. Pipe line 34 which connects compressor 35 to extraction unit12 is equipped with cooling condenser 41.

When the unit is in operation, feed tank 13 is filled with a solution offermentable material 42. The fermentable materials are well known tothose skilled in this art. Such materials are carbohydrate-containingand include blackstrap molasses, invert sugar, sucrose, fructose,glucose, wood sugar, xylose, beet molasses, citrus molasses, hydrol,sulfite liquors, saccharified starches, potatoes, corn, wheat, oats, andother fermentable materials well known to those skilled in this art.

This material is pumped by pump 17 into central distribution pipe 18 ofreactor 11. Surrounding central distribution pipe 18 and within membranewall 22 is contained fermentation liquids 43 which include the culturesthat do the fermentation.

These cultures are also well known to those skilled in this art andinclude Clostridium acetobutylicum (Weizmann), Clostridium butylicum,Clostridium beijerinckii, B. butylaceticum, B. butylicus B.F., B.granulobacter pectinovorum, and others well known to those skilled inthis art. Other cultures are set forth in Chapter 13 of IndustrialMicrobiology by Dunn (McGraw Hill, 1959), the disclosure of which ishereby incorporated by reference.

The filter action of filter medium 23 prevents the bacteria in theculture from passing through and retains them in the space between pipe18 and membrane wall 22. Liquid 44 which passes through membrane wall 22collects in the space between membrane wall 22 and outer wall 26 andthen flows through pipe line 28 into extraction unit 12 at the pointdesignated as 29.

Pump 17 is operated at an appropriate rate to pass the fermentationliquor 43 through reactor 11 at a rate that will cause the fermentationto proceed to a point where somewhat less than the toxification level ofbutanol is reached in the liquor. This level would be up to about 4%,typically from about 1.5% to about 2% by weight. The exact level woulddepend upon the specific culture used.

The liquor which enters extraction unit 12 at the point designated as 29passes up the extraction unit. Countercurrent to the liquor flow,extraction solvent 47 passes in the form of fine droplets generated byexiting from injector units 32. As shown in FIG. 1, it is assumed thatthe extraction operation is conducted with a solvent which is more densethan the mixture of water, butanol, excess sugars, and other solvents.The solvents used in this invention are set forth in detail in U.S. Pat.No. 4,260,836 to Levy, entitled "Solvent Extraction Of Alcohols FromWater Solutions With Fluorocarbon Solvents". The disclosure of thispatent is hereby incorporated by reference. A preferred solvent isfluorocarbon 11 (F-11) (monofluorotrichloromethane) which has a highsolvency for butanol and a very low solvency for water. In addition, thedensity is high (1.464), the boiling point is approximately roomtemperature at one atmosphere pressure (23.8° C.), and it has a low heatof vaporization (43.5 calories per gram). These features, combined withits low flammability, make F-11 an ideal extraction solvent.

Extraction unit 12 may also be operated with solvents such asisopentane, which have a comparatively low density. Isopentane is apreferred solvent in this regard in that it has a specific gravity of0.621, a boiling point of 28° C., a heat of vaporization of 88.7calories per gram, and good solvency for n-butanol and the otherproducts produced by the fermentation. The main difference vis-a-vissolvent density in the extraction unit operation is that, for a lowdensity solvent such as isopentane, the solvent would rise to the top ofthe column so that the entire unit would be essentially inverted. Thisis shown in FIG. 8 where the corresponding parts of the invertedextraction unit 12a are numbered 14a, 29a, 30a, 31a, 32a, 33a, 34a, 35a,36a, 37a, 38a, 40a, 41a, 47a, 48a, 49a, and 50a.

Two differences result from the use of isopentane rather than F-11.First, the heat of vaporization of isopentane is somewhat greater thanthat of the F-11. Second, isopentane is highly flammable which makes theunit more difficult to operate safely. The relative cost of isopentaneis presently much lower than that of F-11 so that the use of isopentanemay be advantageous with larger units which require large amounts ofsolvent material.

Monofluorotrichloromethane and isopentane are merely illustrative of thesolvents that may be used in this invention. Other solvents that havethe appropriate solvency action and that use relatively small amounts ofheat for vaporization may also be used. Furthermore, mixtures of two ormore solvents may also be used. Mixtures of solvents may be devised withparticular properties such as solvency action.

Referring again to FIG. 1, compressor 35 pumps extraction solvent 47into extractor injection units 32 and pulls a reduced pressure in theextraction separation unit 14 which vaporizes extraction solvent 47 atnozzle 40 to deposit mixture 48, which contains n-butanol in the bottomof unit 14 where it is periodically removed via pipe line 50 and valve49. Condenser 41 cools the extraction solvent vapors so that solvent 47is delivered to standing pipe 33 as a liquid.

The extracted liquor passes out of the top of extractor unit 12 intopipe line 38 to feed tank 13 where additional feed material is added tothe liquor via valve 39. Feed tank 13 is equipped with mixer 53 whichhas agitators 54 to conveniently mix recycled liquor 42 with the newfeed material. Carbon dioxide and other gases are generated by thefermentation. These gases are controllably exhausted from reactor 11 bymeans of pipe line 51 and control pressure valve 52. Reactor 11 isequipped with drain 55 in order to selectively or completely remove thecontents thereof.

Compressor 35 is operated at a rate sufficiently high to provide enoughextraction solvent 47 to completely remove all of the butanol from theliquor. This rate is dependent upon the pass rate of liquor through thereactor system.

The cylindrical design of reactor 11 is advantageous in the continuousfermentation process. One advantage results from the fact that thevelocity of recycled liquor 42 is highest when it enters region 43 wherethe culture is present. At that point in reactor 11 there is littlefermentation product to inhibit the fermentation and reduce thefermentation rate and the concentration of the substrate is highestvis-a-vis any other point in reactor 11. Thus, reaction proceeds at ahigh rate. As the liquor moves outward, the velocity decreases, thecross section of available culture increases, and the reaction iscontinued at a high rate because of the longer residence time and thelarger number of bacilli operating on the liquor which compensates forthe inhibiting action of the fermentation products produced. The resultis an efficient fermentation conversion.

There is another advantage to the preferred cylindrical shape of reactor11. At the point where the liquor passes through membrane filter 22, thevelocity is very low compared with the entrance velocity at centraldistribution pipe 18. This avoids the difficulty of clogging filtermedium 23 by having the fluid pass through at high rates.

Clogging may also be reduced by the use of a scraper unit against theinner wall of membrane wall 22 and also by the introduction of apulsating pressure at sonic to ultra sonic frequencies into space 46 bythe use of conventional transducers. The problem associated withclogging of filter medium 23, i.e., decreased flow through filter medium23, may also be overcome by increasing the area of the membrane wall.One means of accomplishing this is illustrated in FIG. 7 where membranewall 22 is pleated at points 56 and 58. This pleated structuresubstantially increases the area of the membrane wall and reduces thepressure drop caused by the flow. This construction may be used withparallel plate constructions of the reactor that would be suitable forsome applications.

Certain culture variations of the bacteria act at different stages inthe fermentation process for converting the carbohydrate-containingsubstrate to butanol. In order to improve the rate and efficiency ofoperation of the fermentation unit, it may be desirable to segregatethese different types of cultures in reactor 11. This may beaccomplished by employing membranes similar to membrane 22, but havingdifferent pore sizes to suit the particular circumstances. Thisarrangement is shown in FIG. 9 by membranes 57.

The apparatus used in the fermentation system described herein may bemade with a variety of materials. These materials should not be affectedby the fermentation process or by the fermentation products. Reactor 11and other equipment may be made, for example, from steel, stainlesssteel, aluminum, polyvinyl chloride plastic, polyethylene, and otherresistant polymers. The porous pipe may be made from sintered stainlesssteel, bronze, aluminum, or other materials known to those skilled inthis art. The membrane filter material may be made, for example, fromcellulose fiber and may be essentially a filter paper. It may also bemade from a synthetic fiber paper using polyester or polypropylenefibers. Furthermore, certain filter membranes are made of polyvinylchloride and of polypropylene. These membranes are made with pores inthe range of from about 0.1 to about 2 microns, which is the normal sizerange for the bacteria spores. Other filter materials, well known tothose skilled in this art, may also be used as the filter material. Theparticular pore size of the filter material depends upon the cultureemployed and may be readily determined by those skilled in this art.

The materials of construction for the other portions of the apparatus ofthe fermentation system described herein are metals which areconventionally used in chemical processing equipment.

Other additives may be used in reactor 11 in order to, for example,adjust the pH of the system to enhance fermentation. Typical additivesto control pH include calcium hydroxide, calcium carbonate, and dilutesolutions of sulfuric acid. The usual amount of calcium carbonate isfrom about 5 to about 7 percent by weight of the substrate.

Nutrients, such as amino acid-containing substances, may also be addedto reactor 11. Ethyl grain, butyl grain, and molasses stillages may beused to buffer the fermentation mass and serve as a source of nutrients.The use of stillage results in better yields. Other additives toincrease yield and control the fermentation rate are set forth inChapter 13 of Industrial Microbiology (McGraw Hill 1959).

The fermentation reaction requires certain conditions. The optimumfermentation temperature is 31° C. Although the pH may vary from about 5to about 7, the initial pH is usually from about 5.5 to about 6.5 whilethe final pH is typically from about 5.2 to about 6.2. Since thefermentation is anaerobic, it may be advantageous to maintain sterilecarbon dioxide at a pressure until the bacteria have had an opportunityto build up a pressure of their own.

The cultures in reactor 11 are preferably contained in a fixed bed. Thetendency of n-butanol in sufficiently high concentrations to inhibit thefermentation reaction is reduced by the presence of the filter memberwhich effectively removes the n-butanol from the culture shortly afterproduction.

The present invention is further illustrated by the following example.All parts and percentages in the example as well as in the specificationand claims are by weight unless otherwise specified.

EXAMPLE

Maize or other similar carbohydrate material is ground to a coarse mealwhich is then mixed with sufficient water to give approximately an 8% byweight concentration of maize. If desired, the corn germ and a portionof the bran may be removed prior to mashing. The resulting mixture ofmaize meal and water is introduced into pressure cookers provided withsuitable agitators, where it is heated with live steam at approximatelythirty pounds pressure for two hours. This operation serves the doublepurpose of thoroughly sterilizing the mash and at the same timeconverting the starch of the maize into a form more easily acted upon bythe bacteria.

Two "cooks" of mash consisting of about 5,600 gallons each, prepared inthe manner described above, are cooled to approximately 37° C. and thenintroduced into reactor 11 which has previously been thoroughlysterilized. The mash is next inoculated with a culture of butylacetonicbacillus, preferably Clostridium acetobutylicum (Weizmann). The amountof inoculum may vary, but it is generally preferred to use about 2% byvolume. At subsequent intervals of about four hours, additional mash isadded in lots of either one or two "cooks" (consisting of about 5,600gallons each) until reactor 11 contains a total of seven "cooks." It isthus seen that twelve hours or more time must elapse before reactor 11reaches its maximum fermenting capacity. A normal fermentation isusually completed in approximately 52 hours from the time ofinoculation. During the course of the fermentation, abundant quantitiesof hydrogen and carbon dioxide gases are liberated and solvents inapproximately the ratio of six parts of n-butanol, three parts ofacetone and one part of ethyl alcohol are formed. At the conclusion ofthe fermentation, the product mixture is extracted as discussed indetail hereinabove.

Further details of the fermentation process, including substrates,cultures, etc. are set forth in U.S. Pat. No. 1,875,536, the disclosureof which is hereby incorporated by reference.

The principles, preferred embodiments, and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in this art withoutdeparting from the spirit of the invention.

I claim:
 1. A continuous process for the production of n-butanol fromthe starting materials of an anaerobic fermentation process consistingessentially of:(a) continuously contacting at least onecarbohydrate-containing substrate with an n-butanol producing culture inwater to effect the fermentation of said substrate and form a productmixture comprising n-butanol, (b) continuously extracting said productmixture from said substrate, said culture and said water by forming asolution of said product mixture with an extraction solvent, whereinsaid solvent is at least one member selected from the group consistingof monofluorotrichloromethane and isopentane, while substantiallyavoiding the formation of a solution of said solvent with saidsubstrate, said culture, and said water, (c) continuously separatingsaid extraction solvent from said product mixture by vaporizingsubstantially all of said solvent without substantial vaporization ofsaid product mixture, and (d) continuously condensing said vaporizedsolvent for reuse as an extraction solvent in step b.
 2. The process ofclaim 1 wherein said culture is of the genus Clostridium.
 3. The processof claim 2 wherein said culture is Clostridium acetobutylicum(Weizmann).
 4. The process of claim 1 wherein said substrate is a mashof starched materials derived from corn, wheat, oats, and other grainmaterials.
 5. The process of claim 1 wherein said substrate is at leastone member selected from the group consisting of beet molasses,blackstrap molasses, citrus molasses, invert sugar, sucrose, fructose,glucose, wood sugar and xylose.
 6. The process of claim 1 wherein saidextraction solvent is monofluorotrichloromethane.
 7. The process ofclaim 1 where said extraction solvent is isopentane.
 8. The process ofclaim 1 wherein said product mixture is separated from said cultureprior to extraction step b.
 9. The process of claim 1 wherein saidfermentation is conducted stepwise by different strains of bacteria. 10.A continuous process for the production of n-butanol from the startingmaterials of an anaerobic fermentation process consisting essentiallyof:(a) continuously contacting a blackstrap molasses substrate with aneffective amount of Clostridium acetobutylicum (Weizmann) culture inwater to effect the fermentation of said substrate and form a productmixture comprising n-butanol, (b) continuously extracting said productmixture from said substrate and said culture by forming a solution ofsaid product mixture with monofluorotrichloromethane solvent, whilesubstantially avoiding the formation of a solution of said solvent withsaid substrate, said culture, and said water, (c) continuouslyseparating said extraction solvent from said product mixture byvaporizing substantially all of said solvent without substantialvaporization of said product mixture, and (d) continuously condensingsaid vaporized solvent for reuse as an extraction solvent in step b.